1. Field of the Invention
The present invention relates generally to a printing apparatus and method, and is particularly concerned with an apparatus and method for printing using an ink composition of the type which is solid at normal room temperature and is rendered liquid or flowable at elevated temperatures.
2. Description of the Prior Art
In many types of product manufacturing and handling operations, particularly food and pharmaceutical packaging, it is necessary to print some type of variable information on the products in addition to the fixed information which appears on pre-printed labels and packaging material. In the case of food and pharmaceutical products, the variable information is very often a date code signifying the date of manufacture or the last date on which the product can be used or sold. The variable information may also consist of a unit price, a lot or batch number, or the like.
Various types of apparatus have been employed for printing date codes and other variable information on products or on the packaging material therefor. This operation is usually referred to as product marking or coding, and the apparatus as a marking or coding device. A typical arrangement consists of a small product marking machine installed as an add-on unit or a conveyor system or other parent machine, the latter carrying either the products themselves or the strip of packaging material which is later cut and applied to the products. It is usually preferred that the marking machine be as simple, reliable and inexpensive as possible. A complex marking apparatus is undesirable from the standpoint of cost and also because it will typically be used and maintained by a product manufacturer who has little or no understanding of its construction or operation. Ink replacement and legend changes, in particular, must be simple to accomplish because these operations may be required very frequently. In the case of date coding, for example, legend changes may be made on a daily basis. Changes in unit prices, lot numbers and batch numbers may be made even more frequently, particularly in manufacturing plants where a number of different types of products are produced. Reliability is important because one wishes to avoid a situation in which a breakdown in the product marking unit forces a temporary shutdown of the entire product manufacturing or packaging operation.
A very useful type of ink composition for product marking applications is one which is solid at normal room temperatures and is rendered liquid or flowable at elevated temperatures. Such an ink composition is kept heated during printing, but it quickly cools and dries after it is applied to the surface to be printed. This makes it possible to handle the printed surface almost immediately without the danger of smearing the printed image. The ink composition is usually impregnated in a cylindrical body of felt or porous plastic foam which serves simultaneously as a supply or reservoir of ink and as an inking or applicator roll for applying the ink to the printing elements. Replacement of the ink supply is accomplished by substituting a new inking roll for the spent or depleted roll. Since the ink composition is solid until it is heated, the spare rolls can be conveniently handled and stored without the risk of ink spillage or mess.
In order to use an ink composition of the type just described, it is essential that the printing or marking device be equipped with means for constantly maintaining the ink in a heated state while it is held on the inking roll as well as after it has been transferred to the printing element. If this is not done, premature cooling and drying of the ink may occur, as for example on the surface of the printing element. This will result in poor print quality. In the past, heating of the inking roll was accomplished in one of two ways. The first method involved heating the shaft on which the inking roll was arranged to rotate, so that heat was delivered to the inking roll by conduction. The second method involved partially enclosing the inking roll in a curved metal heater block with embedded electrical resistance heating devices. This provided a sufficient amount of radiant heat to the inking roll to maintain the ink in a liquid or flowable state. In either case, a separate heating device was required for the printing member in order to prevent cooling and drying of the ink after transfer to the printing elements. This was implemented by means of a separate metal heater block equipped with embedded electrical resistance heating devices and held in contact with the rear faces of the printing elements, the latter usually consisting of metal type. In this way, the printing elements were heated by conduction from the heater block in order to maintain the ink in its liquid or flowable state until contact with the surface to be printed.
Although printing machines constructed along the foregoing lines have proved to be satisfactory for product marking applications, several problems still exist. For example, since the printing elements are heated by conduction from the underlying heater block, the printing elements must be made of a material which is a good conductor of heat but cannot be damaged by direct contact with high temperature surfaces. For all practical purposes, this requires the use of metal type or composite metal-rubber type (e.g., brass-bodied type with rubber typefaces) to the exclusion of less expensive all-rubber type. There is also some difficulty in making electrical connections to the resistance heating devices in the heater block of the printing member, since the latter is almost always a movably mounted component. In cases where the printing member is a cyclically reciprocating arm, rather than a rotary member, direct wire connections can be used as long as there is enough free play in the wires to allow for movement of the arm. In cases where the printing member is of the continuous rotary type, however, direct wire connections cannot be used and one must instead resort to brushes and slip rings or equivalent arrangements. This introduces undesirable complexity and expense into the apparatus as well as additional sources of wear and component failure.